U.S. Pat. No. 4,027,669 describes a thermoplastic syringe barrel with an integrally molded internally threaded locking sleeve at its forward end. The locking sleeve was segmented by three circumferentially spaced slots to provide a sleeve structure with internal threads that could be longitudinally stripped (without unscrewing) from its mold during formation.
During mold stripping, it is desirable to have the sleeve as flexible as possible so the threads are not damaged during the stripping process. During use, just the opposite is desirable, i.e. the sleeve should be as stiff as possible to prevent lateral ears of a hypodermic needle from slipping on the threads. Because of these two divergent needs relative to flexibility of the sleeve, seemingly minute changes in structure can have a tremendous effect on moldability and performance of the syringe. It was previously believed necessary to include a circumferentially uninterrupted portion of approximately 0.100 inch length at a rear of the locking sleeve. This was felt necessary to provide the backup support to prevent overflexing of the sleeve when the needle ears were pushing outwardly against the sleeve during use. An expanded view of this prior art is shown in FIG. 1.